A range detector (10) uses an optical system (11) focused at infinity to form an instantaneous image (13, 113, 213) of an object (16) on a detector (12). As the range between the optical system (11) and an independently moveable object (16) decreases, the image (113, 213) becomes progressively blurred and rapidly increases in size. This blurred image (113, 213) is assessed by detector elements (14, 114, 214) which produce a signal indicating the proximity of the object (16).

A range detector, for assessing the instantaneous range between the detector and an independently moveable object, comprising an optical system arranged to focus an instantaneous image of the object at infinity onto the detector such that the instantaneous image of the object will become progressively unfocused as the range of the object from the detector decreases wherein the detector is arranged to produce a signal dependent on the size of the blur circle associated with the unfocussed image as an indication of the instantaneous range of the object from the detector.

2.

A range detector, according to Claim 1 in which the detector is a staring image sensor.

3.

A range detector according to claims 1 or 2, wherein the detector comprises a plurality of nested detector elements.

4.

A range detector, according to Claim 3, in which the detector elements are positioned to take account of any offaxis image.

5.

A range detector, according to Claim 4, in which at least some of the detector elements are arranged in a ring to produce a signal indicating the direction in which the image is offaxis.

6.

A range detector, according to Claim 4, in which the detector elements are arranged in several concentric rings such that the detectors of each ring can produce a signal indicating the direction in which the image is offaxis.

7.

A vehicle, equipped with a range detector according to any preceding claim, in which the detector is arranged to produce the signal when the range of the object from the vehicle is a predetermined minimum.

8.

A vehicle, according to Claim 7, in which the signal is arranged to actuate a collision avoidance means.

9.

A vehicle, according to Claim 7, and in the case where the vehicle is a missile, in which the signal is arranged to arm a warhead.

10.

A vehicle, according to Claim 8, and in the case where the vehicle is a missile, in which the signal is arranged to detonate a warhead.

11.

A method for assessing the instantaneous range of an object comprising focusing an image of the object at infinity onto a detector such that the instantaneous image of the object will become progressively unfocused as the range of the object from the detector decreases and using the increasing size of the blur circle associated with the image as an indication of the range.

Description:

RANGE DETECTION

This invention relates to the detection of range between a range detector and an independently moveable object, and more particularly to a range detector, to a vehicle equipped with such range detector, and to a method for assessing the instantaneous range of an object.

There are many existing products for measuring range such as radar systems and laser systems. Many collision avoidance systems monitor the road scene ahead and use the rate of growth of the image as an indicator of the distance to a potential obstacle. These systems generally use complex image processing techniques to analyse the dimensions of objects in an image of the scene ahead. The response time of such systems, however, render them inappropriate for detecting the range of a fast closing object such as an aerial target under attack by a missile. This is largely due to the speed of travel involved and the requirement that the missile be detonated as close to the target as possible. Although the ideal scenario is for a missile to impact its target, in practice the target will usually be attempting to evade the missile and it is effective to detonate the missile a few metres away from the target.

In particular, there is a need for a simple and inexpensive range detector for arming and/or detonating a missile such as an air-to-air or a ground-to-air missile, at a range that will cause damage to the target. There is also a need for a simple and inexpensive range detector for assessing the separation of travelling motor vehicles thereby enabling the maintenance of minimum safe separation. According to one aspect of the invention, a range detector, for assessing the instantaneous range between the detector and an independently moveable object, comprising an optical system arranged to focus an image of the object at infinity onto the detector such that the instantaneous image of the object will become progressively unfocused as the range of the object from the detector decreases, and the detector is arranged to produce a signal dependent on the

CONHRMAΉΘN COPY

size of the blur circle associated with the unfocussed image as an indication of the instantaneous range of the object from the detector.

As a consequence of infinity focus, a target object becomes progressively blurred as the range decreases, allowing the increasing size of the blurred image to be used as an indication of the range. This technique facilitates the use of simple staring image sensors to provide an instantaneous image of the object. By staring image sensors is meant sensors where all of the detecting elements are simultaneously exposed to the image from the object, producing outputs continuously and in parallel. Since the increasing size of the blur circle associated with the instantaneous image of the target object, rather than the size of the object itself is used as an indicator of range, minimal image processing is required and response times appropriate for critical applications such as missile systems are achieved.

The detector preferably comprises a plurality of nested detector elements, that is, an array of detector elements comprising a detector element which is surrounded or enclosed by at least one other detector element. The detector elements are preferably positioned to take account of any off-axis image. At least some of the detector elements may be arranged in a ring to produce a signal indicating the direction in which the image is off-axis. The detection elements may be arranged in several concentric rings such that the detector of each ring can produce a signal indicating the direction in which the image is off-axis.

According to another aspect of the invention a vehicle may be provided with the range detector as specified above, the range detector being arranged to produce the signal when the range of the object from the vehicle is a predetermined minimum. Preferably the signal is arranged to activate a collision avoidance means.

In the case where the vehicle is a missile, the signal may be arranged to arm or to detonate a warhead. According to a further aspect of the invention a method, for assessing the instantaneous range of an object comprises focusing an image of the object at

infinity onto a detector such that the instantaneous image of the object will become progressively unfocused as the range of the object from the detector decreases and using the increasing size of the blur circle associated with the image as an indication of the range. The invention is now described, by way of example only, with reference to the accompanying drawings, in which: -

Figures 1 to 3 are diagrams of a range detector illustrating the increase of image size with decreasing range,

Figure 4 is a graph showing the very rapid increase of the blurred image with decreasing range, using f#2 with a 25 millimetre diameter aperture and λ=1μm, and

Figures 5, 6 and 7 are front elevations of alternative configurations of detector elements for use in the range detector of Figures 1 to 3.

In Figures 1 , 2 and 3 a range detector 10 comprises the combination of an optical system 11 and a detector 12.

The optical system 11 is shown diagrammatically as a single convex lens positioned to focus an image of an object at infinity onto a staring detector 12 as shown in Figure 1. Infinity focus is the state where the lens forms an image of an object an infinite distance away and corresponds to the point of focus for parallel rays. The image is formed at the focal point of the lens. The size of the image is indicated by circles 13, 113 and 213. The optical system 11 may comprise any selection or combination of optical elements, such as lenses and/or prisms, and its sole function is to form an image of an object on the detector 12. The staring detector 12 is shown diagrammatically as a plurality of detector elements 14, 114 and 214 supported by a backing plate 15 in a position aligned with the optical system 11. The detector elements 14, 114, 214 may be of any convenient design, for instance electro-optical elements that produce respective electrical signals when subjected to optical or infrared illumination. The use of electro-optical elements to sense the presence, or

absence, of light is very well known in the art and can comprise an array of detector elements 14, 114, 214 such as those indicated in Figures 5, 6 and 7, or a matrix of electro-optical elements.

The operation of the range detector 10 is illustrated in Figures 1 , 2 and 3 In Figure 1 , the object is not shown and will be a substantial distance to the left of the optical system 11 so that it will be focused on the inner detection element 14 as a small image 13.

In Figure 2, the range between the object 16 and the optical system 11 has decreased to an extent that it is no longer focussed on the detector 12, the focal point 17 having moved to the position shown. As a consequence, the image 113 of the object projected onto the detector 12 is fuzzy and the blur circle associated with the unfocussed image of the object additionally extends over the middle detector element 114.

In Figure 3, the range between the object 16 and the optical system 11 has decreased even further, the focal point 17 having moved to the position shown with the consequence that the blur circle associated with the unfocussed image of the object 213 has expanded to extend over the outer detector element 214.

Although only three detector elements 14, 114 and 214 have been shown, a much greater number of detector elements may be used, all detector elements being simultaneously exposed to the image from the object.

Figure 4 illustrates the very rapid manner in which the diameter of the image 13, 113, 213 increases as the range of the object 16 decreases. In particular the diameter of the blur circle associated with images 113, 213 increases several times as the range decreases from 20 metres to 2.5 metres.

When applied to a missile, the range detector 10 provides a very sensitive measurement of the range to target during the very short time before detonation. The detector 12 can be arranged to provide a first electrical signal when the range is only, say, 20 metres to arm a warhead, and to provide a second electrical signal to cause detonation of the warhead when the range has

reduced to, say, 5 metres. Although the missile may have an independent navigation system for following and engaging the target, the range detector 10 can also be used to provide fine adjustment of the missile trajectory by assessing the extent to which the blur circle of the fuzzy image 1 13 or 213 is not centred on the detector 12. In use, the range detector 10 may be positioned to view either directly ahead of the missile to measure the range to the target, or to the side so that the lateral spacing of the missile and target trajectories is measured as the missile overtakes the target.

The detector 12 illustrated in Figure 5 has the three nested detector elements 14, 114 and 214 arranged as a square 14 surrounded by equispaced strips 1 14 and 214, whereas the detector 12 illustrated in Figure 6 has the first detector 14 defined by a circle 14 surrounded by equispaced rings 1 14 and 214. Detectors 12 having the configuration of either Figure 5 of Figure 6 are suitable for assessing the increasing size of the blur circle 213 but do not give any signal as to whether the blur circle is off-axis. For any given optical system, the relative positions of the detector elements 114 and 214 will determine the range at which arming or firing signals are generated. If the detector 12 is only required to give one signal, say for firing, only one of the detector elements 1 14 and 214 is needed. The detector 12 illustrated in Figure 7 has an array of nested detector elements similar to those illustrated in Figure 6 but with the detector elements 1 14 and 214 divided into separate quadrants as shown. In this manner, if the blur circle 1 13 or 213 is off-axis, it will only cover some of the quadrants, or will produce different signals from the different quadrants, thereby enabling a signal to be generated to trim the missile trajectory in the correct direction.

When fitted to a missile, the range detector 10 could of course be disposed to look forward along the longitudinal axis of the missile. However, it is preferred for the range detector 10 to be positioned to view the target through a side wall of the missile, the optical system 11 being arranged to look either forward through a prism to assess the target range along the missile trajectory, or to look sideways to assess lateral separation between the target and the missile.

It will be appreciated that this invention introduces a method that assesses target range by producing a target image such that the target image becomes progressively blurred as the range decreases, and then uses the increasing size of the blurred image as an indication of range. Although the sensitivity of the range detector 10 of the present application renders it particularly suitable for missile applications, it should be appreciated that it is also suitable for use in many other applications where it is important to assess the proximity of an independently moveable object. For instance, docking satellites, the separation of a ship from its dock, the clearance of a motor vehicle from the walls of its garage, and particularly for maintaining safe longitudinal and lateral spacing between motor vehicles travelling along a carriageway.